1. Technical Field
The present invention relates to a field effect transistor made of a group III nitride semiconductor.
2. Related Art
A MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) using GaN material and a HEMT (High Electron Mobility Transistor) using AlGaN/GaN material are known as field effect transistors using a group III nitride semiconductor, as shown by Non-Patent Documents 1 and 2 and Patent Document 1, for example. These devices have higher dielectric breakdown voltage (referred to hereinafter as “withstand voltage”) and higher saturation mobility (referred to hereinafter as “mobility”) than field effect transistors using other group III compound semiconductor materials such as Si, GaAs, and InP. Furthermore, these devices perform normally-off operation, and are therefore suitable for power devices.    Non-Patent Document 1: M. Kuraguchi et al., “Normally-off GaN-MISFET with well-controlled threshold voltage”, International Workshop on Nitride Semiconductors 2006 (IWN2006), Oct. 22-27, 2006, Kyoto, Japan, WeED1-4.    Non-Patent Document 2: Huang W, Khan T, Chow T P: Enhancement-Mode n-Channel GaN MOFETs on p and n-GaN/Sapphire substrates. In: 18th International Symposium on Power Semiconductor Devices and ICs (ISPSD) 2006 (Italy), 10-1.    Patent Document 1: International Publication WO 2003/071607
Among conventional GaN MOSFETs, devices have been announced that have high mobilities of 167 cm2/Vs or high withstand voltages near 1000 V, but a field effect transistor that has both high mobility and high withstand voltage has yet to be realized.
In light of this, it is an objective of the present invention to provide a normally-off field effect transistor that has both high mobility and high withstand voltage